Pontiac Grand Prix

Front Rear AllWheel Drive Road Test

No Grip, No Glory: One is the logical choice; one is for speed freaks; and the other is for those who find passion behind the wheel

My screen blinked full of matches when I typed "rear-wheel drive" into Google--among them, www.rearwheeldrive.org. Created by Parker Thomas, the site's a simple but delightful online shrine to rear drive. When Thomas was visiting Australia a few years ago, he was bowled over by the multitude of fun-handling rear-drive cars still available in that lost world of drivetrains. His site grew out of the epiphany and includes this, my favorite bit from his Drive Line Quiz:

Question 7: Which type of vehicle accelerates and handles best while driving backwards?A. Front-wheel drive.B. Rear-wheel drive.Answer: A. Front-wheel drive, since it's functioning as rear-wheel drive at the time!

Attaboy...Thomas's dream seems set to come true. Rear drive is in vogue right now, with a roster of car manufacturers pregnant with 300- and 400-horsepower rear-drive cars they'll soon be hawking at dealerships near you. Here's crossing our fingers that stability control will protect us from all those sweaty-fingered young drivers who've never operated a rear-drive car.The current chic allure of rear drive should be amusing to anyone older than 30. Three decades ago, Congress enacted the first Corporate Average Fuel Economy standards in response to the 1973 oil crisis, and Detroit's reaction was to push the go button for front drive. Its lighter weight and denser packaging--long evolved in Europe--looked like part of the answer, allowing, as it did, smaller and lighter cars while retaining much of the interior volume super-sizing Americans delight in. But the second oil spike of 1979 wasn't followed by a third. Prices ebbed and worries eased. As Chrysler's Tom Gale said in a 1999 article written by Bill Visnic in Ward's Auto World, "We don't have the political will to do something about the (unnaturally low) price of fuel," adding that without it, "you'll have people driving around in Kenworths." With memories fading, the fuel-economy penalty of rear- and all-wheel drive, relative to front drive, is now rarely mentioned.But what's behind their differences for the enthusiast driver? To investigate, we rounded up a trio of potent, 3000-horsepower (or so) sedans that exemplify each type. Like kids with magnifying glasses and three flies pinned to the table, we tortured them for answers around the skidpad, sifted through technical documents, and handed their keys to a driving expert at the Bob Bondurant School of High Performance Driving.Representing rear drive is the latest Infiniti G35 6MT sedan. The headline here is that the pop from Nissan's latest 3.5-liter V-6 is up 38 horsepower (to 298) as installed in our six-speed manual car (the automatic rises 20). And it's owed to nothing more than revised exhaust timing, beefier internals, dialed-up cooling, and heightened airflow. Garnished with bigger brakes and simmered in revised coachwork (updated hood, trunk, bumpers, headlights, and twin circular taillights), Infiniti's sedan-du-jour is a dynamite ride, though the ride quality of the new 45-series tires on 18-inch wheels can get jittery.Front drive is swaggeringly personified here by the somewhat garish but functionally impressive new GXP edition of the Pontiac Grand Prix. Inheriting the GXP moniker from the soon to evaporate Bonneville line, the pumped-up Prix has grabbed an additional handful of steroids courtesy of a 303-horsepower, 5.3-liter Vortec V-8, thus creating the first V-8-powered GP in decades. How do we say this sedan isn't for just anybody? From the moment the engine fires, the exhaust throbs like you're on your way to the malt shop in high school again. And the interior is festooned in more ornamentation than a Mardi Gras reveler. However, the beast finds focus when you crack open the throttle. 60 mph? Punch it to the carpet for six breathless seconds and hang onto the steering wheel as it wriggles with spasms of torque. When not mussing the hair of innocent bystanders, the 5.3 tries to act civil, lapsing into a nearly invisible fuel-saving displacement-on-demand mode.Last, the all-wheel-drive flag is waved by a 300-horsepower Subaru WRX STi--and not just any STi; this silver specimen had already made friends around our offices when it took part in Chris Walton's STi versus Mitsubishi Evolution MR showdown (October 2004). With its sticky tires, adjustable center differential, water-spray for the intercooler, raucous interior noise, and "Mars Attacks" styling, the STi's hardly comparable with the other two in the marketplace, but it's just about the zenith in all-wheel-drive performance.BasicsHow often have you heard someone say this about rear drive: "It just makes sense--the front wheels should steer a car, and the rears propel it." Sounds logical, but it's Homer Simpson simplistic.Agreed, the front tires initiate turning, but their role in cornering is unexpectedly transient. When you dial in a smidge of steering-wheel angle, what happens is that the front tires destabilize the car into a momentary microspin. Fortunately, for pedestrians, the rear tires quickly acquire their own slip angle (and lateral force), and with both ends of the car biting, the whole shebang safely rounds the corner. Notice that all the tires find employment in the act of handling--not just the fronts.Let's hold our nose-heavy Grand Prix GXP up to the light as an example of weight distribution's role in steady cornering. With 63.4 percent of its weight squatting on its front tires (there's a large chunk of V-8 powertrain atop them), the Pontiac's front rubber needs to supply that same 63.4 percentage of the GXP's total lateral grip to negotiate Dead Man's Curve.The unsatisfying way to trace the turn would be to just crank in extra steering and flog the front tires into their 63.4 obligation, twisting in earplugs to endure the sickening understeer. Another would be for its designers to subtly increase the front tire's size, lessening their contact patch's pressure. It's a peculiarity of tires that the effectiveness of a tire's footprint increases with shoe size (lowering contact patch pressure is precisely the reason why racing cars always fit the biggest permissible rubber). Any wonder then that the GXP wears 255/45R18 fronts and 225/50R18 rears?The location of the componentry's weight also affects a car's dynamic handling. Rear drive's lack of compactness means its weight is better distributed--good for braking and cornering. But did you know that engineers often have to crank in a safety net of intentional understeer to accommodate the weight of passengers and cargo, which shifts the center of gravity rearward? Not a problem with nose-heavy front drive.Last, remember that old argument for front drive: "It's better to pull a wagon around a corner than push it"? Neglecting slip angles when rounding a corner, the front and rear wheels are all tangent to the instantaneous center of the turn (although at slightly different radii). All that's being pushed here is a certain cheap deli meat.The Problem of Front Drive And Steering FeelCribbing from pal Doug Milliken and dad Bill's book "Racecar Vehicle Dynamics" (www.sae.org), I have to share the following front-drive nugget: Did you know it was cigar-chomping Barney Oldfield's front-drive Christie that precipitated the expression "plow" (as in, really bad understeer)? Spectators observed that the car was literally plowing ruts as it circled an early dirt racetrack. Another is the observation that there's never been a front-drive racer with more than 300 horsepower successful in Formula One or at Indianapolis (indeed, one of the fatal accidents involving a 500-plus horsepower Novi at Indy has been blamed on terminal understeer).Today, the nasty temperament of early, high-horsepower, front-drive road cars has been largely corralled with equal-length halfshafts and better control of the engine's gyrations. But even the best of them check out on steering feel as the gs go up. Why?There's a geometry problem. The steering feel we relish in good rear-drive cars is traceable to the steering's fidelity in feeding back the tiny torques arising from the suspension's scrub radius and mechanical and pneumatic trail. These small, but critical distances between the steering axis (kingpin axis) and the tire contact patch's center of pressure are what distinguish an Austin-Healey Bugeye Sprite's chatterbox steering column from a garbage truck's (see diagram). The scrub radius subtly informs about road irregularities, the two trails telegraph how hard we're cornering (particularly the pneumatic part that diminishes with lateral load).Apply power to drive a wheel possessing this elegant geometry, and our eloquent Dr. Jekyll would degenerate in a torque-steering Mr. Hyde. What's best for steering feel and what's optimal for torque-steer riddance are simply at loggerheads with one another. To squelch torque steer, the scrub radius needs to be minimized (but not eliminated; it's helpful in keeping the car straight when one wheel spins). Meanwhile, the quiet messages of pneumatic trail are scrambled by throttle application, their intelligence about cornering forces encoded into nonsense. The best systems are just the least painful compromises.What a Driving Expert Has to Say Over the nonstop tire howls that are the Bob Bondurant school's perennial soundtrack, seasoned instructor Mike Speck chuckles, "I was watching a Busch race the other day, and it seemed I'd taught nearly half the drivers in the field." Versatile, Speck has driven them all and knows every trick in the book--which he apparently keeps on a flash memory card implanted in his skull.Although the Bondurant complex features lots of driving-course configurations, we settle on a simple loop to focus our handling query. Moreover, just one left-hand corner: brake, bend counterclockwise into it, trade speed for lateral g, punch the apex button, and unwind the wheel as you're ladling in throttle. Elementary. Except Speck hurtles through it at a comical pace, more like a banking fighter pilot arching a low pass punctuated by touch and goes. And he's casually talking the entire time. "My line is virtually the same for all three cars"--surprising me--"it's simply the fastest path." At the moment, he's hustling the GXP, negotiating with its dominant understeer gene a respectable corner exit strategy. "Because of the understeer, you want to delay turning in--and then turn in quicker to get more yaw into the car. You'll need that rotation on the exit."The opposite strategy is applied to the G35: "Here you want an earlier, slower turn-in to reduce the car's yaw rate; it'll want to rotate plenty on its own throughout the exit."But," he continues, "--this is important--you also need enough steering wound in at the apex to have enough to unwind all the way out to the exit, balancing the tail."Aboard the WRX: "Isn't this a fantastic machine?" Speck's question answers itself as we blister through the left in the turbo-huffing Subaru. "The tires' grip just rockets the car out of the corner"--I'm bracing myself against 1.0 g of lateral acceleration--"that's the huge difference here; just squeeze in the power and go.""Do you ever left-foot brake [massaging both throttle and stop pedal to rotate the car with the rear brakes]?" I mention this because it's a common driving technique among rally drivers."Nah," says Speck, shrugging. "Not in road racing. I've driven with guys who do, and, frankly, I've usually been faster. Left-foot braking is helpful for rotating a ground-effect car's chassis without upsetting its aerodynamically critical pitch, but I've never found much advantage in it."Speck seems exhilarated by the all-wheel-drive WRX, pleasantly surprised by the rear-drive G35, and impressed by how rudely the front-drive GXP can be manhandled. Which goes to show that good engineering--whatever wheels are driven-- goes a long way.Is one better than the other? A thinking engineer would cagily answer, "it depends." Front drive is the Spocklike logical solution in low-power compact cars with high-mileage ambitions. Speed freaks swear by the Velcro traction of all-wheel drive, particularly if their engines are percolating more power than their tires can pour. And rear drive is the choice for passionate drivers.For those of us too clumsy to play basketball and too thick-fingered to play the violin, a rear-drive car's steering wheel is a rare conduit to touch subtlety and balance, delicacy and finesse--even if it's just on the way home from work; the sort of thing that can motivate a businessman visiting Australia to create a Web site.You're probably familiar with the terms "understeer" and "oversteer"--but what do they mean? It's easy to demonstrate on our 200-foot-diameter skidpad. Start by driving slowly (with no tire slip at all) to discover the small steering angle needed to just follow the circle. "Neutral" handling means that as you slowly increase your speed--and lateral g--additional steering angle doesn't need to be cranked in; the car automatically skews slightly to produce more grip. If more steering is required as speed rises, the car is understeering; oversteer demands that you dial some out to avoid spinning.In the illustration, all three of our test cars are understeering while circling at a constant speed (here, "front-slip angle" means added steering angle). Not surprising, as basically all cars understeer--but, contrary to perceptions, the Subaru is doing so more than the nose-heavy Pontiac. So why does the GXP seem more understeery?The trouble is that it doesn't respond much to changing its throttle opening. The bottom image represents how our cars reacted to lifted or full throttle after two seconds have elapsed (without changing steering-wheel angles). The Pontiac hardly reacts at all (if anything, it relentlessly points away from the corner); the STi seems eager to redirect itself; the Infiniti, almost too much so. It's in these instances that the effect of drivetrain layout on vehicle handling is evident.It should be mentioned that all these tests were performed with electronic traction aids switched off as much as possible. With them activated, the G35's full-throttle behavior was transformed: It simply migrated out to a bigger circle diameter (instead of spinning). The Pontiac's antics also were muted. All of which raises the question of what's more significant to vehicle dynamics these days--hardware or software?Steered WheelViewed from the front, the scrub radius--distance between the tire's contact patch and the steering axis--results in tiny torques that can find themselves transmitted back through the steering wheel. With rear drive, this translates into road feel; with front drive, the power being transferred through the contact patch causes torque steer. A front-drive car's scrub radius is commonly negative (shown here) allowing a spinning wheel's lack of grip to be somewhat counteracted by its (gripping) opposite, which automatically aims itself slightly outward.
In side view, two other small leverages affect steering feel: mechanical trail aids straight-line stability, but also provides useful information about cornering forces. The pneumatic trail is an additional leverage, caused by a fore-aft displacement of the tire contact patch's center-of-force. During hard cornering in a rear-drive car, this distance diminishes, telling the driver the limit is near. Alternately, power transferred by front drive forcibly changes the pneumatic trail, muddling the steering message.

If you checked out the cover of October’s Motor Trend magazine, then you already had the inside scoop on the car from Down Under that is more than likely going to become the next Pontiac Grand Prix.And according to Australia’s Drive website, it’s a done deal.